1. Field of the Invention
The present invention relates to an anti-skid brake system for wheeled vehicles.
2. Discussion of the Prior Art
In general, a conventional anti-skid brake system of a wheeled vehicle includes a set of speed sensors arranged to detect each rotational speed of the vehicle road wheels, an electronic control apparatus arranged to be applied with electric signals indicative of the detected speed of the respective road wheels for producing a control signal in accordance with conditions of a travel road of the vehicle, and an electrically operated actuator arranged to be activated under control of the electric control apparatus for controlling a hydraulic braking pressure applied to the wheel brake cylinders in response to the control signal. In the braking operation of the vehicle, the electric control apparatus starts to produce the control signal therefrom when the detected speed of the respective road wheels becomes lower than a reference road wheel speed while each acceleration of the road wheels becomes smaller than a predetermined negative value. If the vehicle passes over a stepped portion on the travel road during a braking operation, the road wheels are instantaneously floated from the road surface, resulting in a sudden decrease of torque acting on the road wheel tires. In such an instance, the road wheel speed becomes lower than the reference road wheel speed, and the acceleration of the road wheels becomes smaller than the predetermined negative value. As a result, the actuator is activated under control of the electric control apparatus to decrease the hydraulic braking pressure applied to the wheel brake cylinders. Accordingly, the driver experiences an unpleasant feel due to a sudden change of the vehicle deceleration during the braking operation.
To avoid such a problem as described above, Japanese Patent Laid-open Publication No. 2-249752 discloses an anti-skid control method for increasing the hydraulic braking pressure after lapse of a predetermined time in a condition where a deceleration gradient of an estimated vehicle speed at an initial control cycle is less than a predetermined value and where a period of time between low and high peaks of the road wheel speed is shorter than a predetermined time. In this control method, however, the hydraulic braking pressure would not be increased during the braking operation if the deceleration gradient of the estimated vehicle speed was higher than the predetermined value, and it would be erroneously increased if the road wheel speed greatly changed due to vibration of the driving system of the vehicle during the braking operation on a road surface of low frictional coefficient. This results in an increase of the slip rate of the road wheels during the braking operation.